Process of bleaching chemical wood pulp with nitrogen trichloride



PROCESS OF BLEACHING CHEMICAL WOOD PULP WITH NITROGEN TRICHLORIDEApplication February 25, 1955 Serial No. 490,679

3 Claims. (Cl. 8-105) No Drawing.

This invention relates to a method of bleaching chemical wood pulp, andin particular to a bleaching process employing a nitrogen trichloridetreatment.

The bleaching of wood pulp, and other cellulosic fibers to a highbrightness, with the common chlorine bleaching agents, such as chlorinewater or hypochlorite, is almost always accompanied by a degradation ofthe cellulose; consequently, the increase in brightness is secured atthe expense of loss in strength of the pulp. Numerous proposals havebeen made relating to means of avoiding this degradation duringbleaching. But in spite of the large amount of work that has been donein the field of pulp bleaching, and of the development of multistagebleaching and its various modifications, no completely satisfactoryanswer to bleaching without degradation has been found. The chloritesand chlorine dioxide do have the property of bleaching withoutdegradation but their high cost is a serious drawback.

Some of the proposals to avoid degradation involve the use of nitrogencompounds. One such proposal (Dodson, U.S. Patent No. 2,478,379) teachesthat the presence of a small amount of an ammonium compound in achlorine bleaching step serves to inhibit degradation of the cellulose;it is said that the amount of ammonium compound present can be only avery small fraction of that necessary to react with all chlorine;otherwise, the beneficial eifects of the presence of the ammoniumcompound are no longer obtained. This particular proposal is essentiallya chlorine bleaching step carried out at a low pH in the presence of avery small amount of an ammonium compound inhibitor.

Chloramines, which are formed by reacting ammonia and chlorine, are saidto be inferior bleaching agents and, according to the literature, theirpresence in any substantial quantity in a bleaching operation isundesirable. Also the technical literature teaches that in bleachingwith a chlorine solution, it is necessary to raise the pH of thechlorine solution to above 4.5 by the addition of one of the alkalies,including ammonia, in order to prevent weakening of the fibers.

However, I have found that there is a certain critical range inproportions of nitrogen compound and chlorine, combined with aparticular pH range, not previously disclosed, wherein very desirablebleaching results are obtained.

In contrast to these prior proposals I have discovered that the reactionproduct (or products), formed when all of the chlorine in an aqueouschlorine solution is reacted with an ammonium compound at a pH below4.5, is an effective bleaching agent, and that little or no weakening ofthe fiber accompanies the bleaching. That is, I have discovered that avery substantial increase in brightness can be secured in a chemicalwood pulp by treating it with nitrogen trichloride in the presence ofwater at a pH below 4.5, and that the increase in brightness isaccomplished with very much less degradation of the cellulose than withthe common bleaching agents. The values for the disperse-viscosity andthe ice bursting and tear strengths of the pulp will be substantiallythe same after bleaching as before, where nitrogen trichloride is used,whereas where chlorine water or a hypochlorite is employed there isusually a substantial loss in strength. The treatment can be carried outby immersing the pulp in an aqueous solution of nitrogen trichloride orby exposing a suitably water-wetted pulp in permeable form to nitrogentrichloride vapors.

My bleaching agent is conveniently prepared by adding a sufiicientamount of an ammonium compound to an aqueous chlorine solution at a pHbelow 4.5 to react with all of the chlorine to form nitrogentrichloride. The reaction is believed to proceed as follows, takingammonium sulfate as a typical ammonium compound:

One part by weight of chlorine is equivalent to approximately 0.62 partby weight of ammonium sulfate. I have found that the presence of freechlorine with the nitrogen trichloride is detrimental, in thatdegradation of the cellulose is pronounced. Consequently, it isnecessary to have present at least one reacting weight of ammoniumcompound for each reacting weight of chlorine; in fact, it is desirableto use a slight excess of ammonium compound over the theoretical amountrequired to react with all of the chlorine present. Various ammoniumcompounds have been found suitable, particularly ammonia itself and thesalts such as the sulfate, chloride, phosphate, and persulfate; salts ofpolyvalent acids are preferred.

While it is most practical to form the nitrogen trichloride in situthereby avoiding the hazards of handling nitrogen trichloride, and toform it most economically from chlorine and an ammonium salt theinvention also applies where the nitrogen trichloride is otherwisederived. For example, the bleaching solution can be prepared by reactingat a pH below 4.5, hypochlorous acid and ammonium sulfate, or sodiumhypochlorite and ammonium chloride. Also, the nitrogen trichloride asthe isolated compound can be added to water to form a bleach solution orit can be vaporized and the gas mixed with wetted cellulose at a pHbelow 4.5.

The pH during the nitrogen trichloride bleaching operation is maintainedbelow 4.5. If necessary, the mixture of chlorine and ammonium compoundis adjusted to a pH below 4.5 by the addition of an acid; if the pulp isalkaline the suspension thereof should be acidified to a pH below 4.5before the nitrogen trichloride solution is added. At higher pHsapparently the compound tends to be converted into dichloramine withless increase in brightness and the fiber is attacked with a loss indisperseviscosity. Good bleaching is obtained at pH values as low as lbut a pH range of 1.5 to 3.0 gives optimum results. Any of thenon-oxidizing acids can be used to secure the desired pH value, forexample, hydrochloric acid, sulfuric acid, phosphoric acid, oxalic acid,etc.

While the nitrogen trichloride bleaching can be carried out in variousways and in place of conventional agents, it is preferably carried outas one of the steps of a conventional multi-stage bleaching process.This agent produces a bleaching effect no matter at what stage in thebleaching process it is applied, and without any material impairment ofthe viscosity. But the non-degrading properties of nitrogen trichloridecan be used to best advantage by employing it in one of the final stepsof a multi-stage process since at that point the cellulose is in a morehighly refined condition and is more susceptible when it is used inplace of hydrochlorite in the final stages. While nitrogen trichloridecan be used in place of other bleaching agents, it works to bestadvantage when used in only one or two of the stages of a multi-stagebleaching and other. conventional bleaching agents, such as chlorine,hypochlorite, chlorine dioxide, and peroxide, are employed in the otherstages.

My invention is particularly suitable for production of high strengthpulp.

Nitrogen trichloride bleaching is aided somewhat by elevatedtemperatures; temperatures of 120 to 160 F. speed the reaction. However,the nitrogen trichloride tends to vaporize so that a closed vessel isadvantageous. Conventional bleaching times are adequate. Pulpconsistency does not appear to be critical.

Usually a solution containing from 0.5% to 1% of available-chlorine,based on the dry weight of the pulp, and converted into nitrogentrichloride with an ammonium compound at a pH below 4.5, will produce asubstantial increase in brightness of the pulp. In some stages as muchas 2.5% (calculated as available-chlorine on the dry weight of pulp) isapplied. Larger amounts can be used but the gain is brightness is notcommensurate with the added cost.

The pulp viscosities given herein were determined by the pipet-versionof Testing Method T 230 sm-46 of the Technical Association of the Pulpand Paper Industry, entitled Cupriethylenediamine Disperse Viscosity ofPulp. The viscosity results secured by this, as well as by otherviscosity methods, are accepted as being indicative of the strength andparticularly the change in strength occurring during a bleaching processor other chemical treatment. As the viscosity of a given pulp drops inthe bleaching process, its strength properties such as tear, tensile,and bursting strength generally will be found to be reduced.

My invention is further illustrated by the following examples:

Example 1 Thirty grams of semi-bleached southern pine sulfate pulphaving a brightness of 73.9, and having a viscosity of 8.3 centipoiseswere suspended in water and to this was added a previously formedmixture of 1.87 cc. of a solution of ammonium sulfate and 76 cc. ofchlorine water containing 0.3 gram of available-chlorine (1% on the dryweight of the pulp). Sufiicient water was then added to form a pulpsuspension of 10% consistency and this was thoroughly mixed. The pH was2.1. The slurry was gradually heated to 120 F. and after 2 hours contactwith the nitrogen trichloride solution the pulp was water washed.Brightness was 82.4 and the viscosity was 7.9 centipoises.

A second portion of the same pulp was bleached with chlorine watercontaining 1% of available-chlorine on the dry weight of the pulp undersubstantially the same conditions of 10% consistency, pH of 2.6,temperature raised to 120 F., and total bleaching time of 2 hours. Itsbrightness was 82.3 but the viscosity had been reduced to 4.9centipoises.

Example 11 Unbleached pine kraft pulp, having a permanganate number of17.9 (Tappi Method T214M-42) and a viscosity of 25.4 centipoises, waspre-bleached by conventional chlorination, caustic extraction, andcalcium hypochlorite steps. Thereafter the bleaching was completed bythree steps consisting in 1) applying 1% of available-chlorine ascalcium hypochlorite at 10% consistency and a temperature of 105 F. for60 minutes, (2) extracting with caustic, and (3) applying 0.6% ofavailable-chlorine as calcium hypochlorite. The bleached pulp had abrightness of 82.7, a viscosity of 9.9 centipoises, an average burstingstrength of 181, and an average tear strength of 564.

A second portion of the same unbleached pulp was bleached by exactly thesame steps, using the same total amount of available-chlorine exceptthat the next to the last hypochlorite step, i.e. (1), was replaced by anitrogen trichloride step. The nitrogen trichloride solution wasprepared by forming a chlorine solution containing 1% available-chlorine(based on the weight of oven dry pulp to be bleached), then addingammonium sulfate in the proportion of 0.62 pound for each pound ofavailablechlorine present. The bleaching with this solution was thencarried out at 10% consistency, a pH of 2.0 to 2.1, and a temperature ofF., for 60 minutes; the process was then completed by a causticextraction and a final hypochlorite bleach carried out under the sameconditions as used on the sample in the preceding paragraph. Thebleached pulp had a brightness of 82.9, a viscosity of 11.2 centipoises,an average bursting strength of 189, and an average tear strength of604.

Thus, it will be seen that nitrogen trichloride can be used in place ofhypochlorite in a multi-stage bleaching process. An equal or betterbrightness is produced with the nitrogen trichloride employing the sameamount of available-chlorine; in addition the pulp is less degraded,exhibiting a higher viscosity and higher bursting strength and tearthroughout the beating curve.

Example III Southern pine sulfate pulp partially bleached byconventional bleaching methods and having a brightness of 68.7 and aviscosity of 11.2 centipoises was further bleached by means of 0.5% ofavailable-chlorine on the weight of the dry pulp applied as nitrogentrichloride. The nitrogen trichloride solution was prepared by adding tochlorine water 0.62 pound of ammonium sulfate for each pound ofavailable-chlorine present and adjusting the pH to 2.4 with sulfuricacid. The nitrogen trichloride bleaching was carried out at 5%consistency and a temperature of F. for minutes, followed by a waterwash. The pulp was found to have a brightness of 78.3 and a viscosity of11.1 centipoises.

Example IV A portion of the same partially bleached pulp used in ExampleIII was exposed to nitrogen trichloride vapors. The pulp was firstdispersed in water acidified with sulfuric acid to a pH of 2.2, thenfiltered, and pressed to a cake having 21.2% solids. This wet cake wasbroken up and placed upon a porous support over an aqueous solutionevolving nitrogen trichloride vapors in a closed chamber. After an hoursexposure, the pulp was washed with water; its final brightness was 76.8as compared to 68.7 before treatment with nitrogen trichloride.

The time of exposure to nitrogen trichloride vapors is dependent uponthe degree of bleaching desired and the concentration of vapor. Normallya substantial degree of whitening is obtained in the first hour ofexposure to vapors which are predominantly nitrogen trichloride, butseveral hours may be needed to secure sufficient absorption if dilutedvapors are employed. No apparent injury is incurred if the pulp remainsin contact with the nitrogen trichloride for several hours. The amountof water need be only enough to moisten the fibers thor oughly anduniformly. A pulp mass containing 50% moisture can be bleached by meansof the vapor. However, since nitrogen trichloride has a somewhat limitedsolubility in water, a higher moisture content, in the range of at least60% to 70%, facilitates absorption of the vapors.

Although in the examples softwood pulps have been employed, theinvention is not limited to coniferous pulps. Nitrogen trichloride isalso adapted to bleaching of hardwood pulps, and can be used in place ofchlorine or hypochlorite, particularly in multistage bleaching. Hardwoodpulps of high brightness can be produced with less degradation utilizingnitrogen trichloride than when using conventional chlorine bleachingagents.

Cellulosic materials other than wood pulp, for example,

cotton and flax, suitably pretreated with alkaline liquors to removewaxy impurities, can also be bleached by the herein described process.

Brightness values given herein were measured on a General Electricbrightness meter.

Bursting strength values given were determined by measuring the burstingstrength on a standard Mullen tester on hand-sheets made from samples ofpulp taken after 0, 25, 50, 75, 100 and 125 minutes ball milling,multiplying the Mullen reading in each case by 100 and dividing by thebasis weight (17 x 22500), and averaging the results. Values given areas percent points per pound.

Tear strength values given were determined by measuring tear strength ona standard Elmendorf tester on handsheets made from samples of pulptaken after 0, 25, 50, 75, 100 and 125 minutes ball milling, multiplyingeach tear reading by 100 and dividing by the basis Weight (17 x 22500),and averaging the results. Values are given as percent points per pound.

I claim:

1. In the process of bleaching chemical wood pulp, the improvement whichcomprises applying nitrogen trichloride to chemical wood pulp in thepresence of a quantity of water at least equal in weight to the dryweight of the pulp, in the absence of free chlorine and at a pH in therange of 1.0 to 4.5 obtained by the addition of a nonoxidizing acid, theperiod of contact between the nitrogen trichloride and the pulp beingsuflicient to permit bleaching to be effected.

2. In a multistage process of bleaching chemical wood pulp, the stepwhich comprises applying nitrogen trichloride to partially bleachedchemical wood pulp in the presence of a quantity of water at least equalin weight to the dry weight of the pulp, in the absence of free ch10-rine and at a pH in the range of 1.0 to 4.5 obtained by the addition ofa non-oxidizing acid, the period of contact between the nitrogentrichloride and the pulp being suflicient to permit bleaching to beefiected.

3. Process of bleaching chemical wood pulp which comprises exposingchemical wool pulp mixed with at least an equal weight of water tonitrogen trichloride vapors in the absence of free chlorine for a periodsuflicient to permit bleaching to be eflected, said mixture of pulp andwater having a pH in the range of 1.0 to 4.5 obtained by the addition ofa non-oxidizing acid.

References Cited in the file of this patent UNITED STATES PATENTS1,957,938 Campbell et al May 8, 1934 2,428,331 Hutchinson Sept. 30, 19472,478,379 Dodson Aug. 9, 1949

1. IN THE PROCESS OF BLEACHING CHEMICAL WOOD PULP, THE IMPROVEMENT WHICHCOMPRISES APPLYING NITROGEN TRICHLORIDE TO CHEMICAL WOOD PULP IN THEPRESENCE OF A QUANTITY OF WATER AT LEAST EQUAL IN WEIGHT TO THE DRYWEIGHT OF THE PULP, IN THE ABSENCE OF FREE CHLORINE AND AT A PH IN THERANGE OF 1.0 TO 4.5 OBTAINED BY THE ADDITION OF A NONOXIDIZING ACID, THEPERIOD OF CONTACT BETWEEN THE NITROGEN TRICHLORIDE AND THE PULP BEINGSUFFICIENT TO PERMIT BLEACHING TO BE EFFECTED.